1. Field of Invention
The present invention relates to solid state laser amplifiers pumped by semiconductor laser diodes and, in particular, to a laser amplifier operating using a wedge configured laser active slab to reduce the generation of amplified spontaneous emission within the laser amplifier.
2. Description of the Background Art
It is known in the relevant art to use laser emission from one or more semiconductor diode lasers to pump a solid-state laser medium (i.e., diode pumping). Diode pumping provides for high power solid-state laser devices that are more efficient, more compact, more reliable, and that have better beam quality, than lamp-pumped solid-state lasers of comparable output power. Many new diode-pumped solid-state laser (DPSSL) designs have emerged in recent years due to an increasing array of new solid-state laser materials, and a wider range of diode laser pump wavelengths from which the laser designer can select.
Various types of semiconductor diode lasers have become popular for pumping solid-state lasers. These include discrete single-emitter diode lasers as well as one-dimensional (1-D) diode laser arrays (i.e., diode “bars”) in which multiple diode laser emitters are integrated onto a single bar of semiconductor material. Two-dimensional (2-D) laser diode arrays incorporate multiple diode laser bars, packaged one above the other, to create a 2-D array of diode laser emitters. Since the emission of the individual diode emitters produce relatively large beam divergence, packaged diode lasers, diode bars, and 2-D diode arrays may include micro-optics to collimate one or both axes of the spatial emission pattern from each diode emitter. In some applications, it may be desirable to couple diode pump light into one or more optical fibers for delivering pump light to the solid-state laser medium.
Diode-pumped solid-state lasers are conventionally categorized as being either end-pumped or side pumped lasers. In end-pumped laser designs, diode laser radiation used for optical pumping travels substantially parallel to the laser beam being generated, or amplified, in the solid-state gain medium. In side-pumped laser designs, diode laser pump radiation travels substantially perpendicularly, or orthogonal, to the laser beam being generated or amplified. End-pumped DPSSLs are typically very efficient (e.g., slope efficiency may be 50% or more), and exhibit very good TEM00 (diffraction-limited) beam quality. End-pumped lasers designs are generally regarded as being limited to generating relatively low average output power levels, due to the possibility of thermal fracture of the solid-state laser medium when diode pump power is increased beyond a specified level. End-pumped DPSSL designs that allow pump light to penetrate deeply into the solid-state medium, and that distribute pump power more uniformly along the entire length of a laser rod or slab, are exceptions, but these lasers often suffer from degraded output beam quality unless certain preventive design measures are implemented.
In contrast, side-pumping methods enable the pumping of solid-state laser media with much higher power levels before thermal fracture occurs, mainly because the diode pump power is distributed over a larger area of the solid-state crystal surface. Historically, side-pumped DPSSL designs have been somewhat less efficient, and have exhibited poorer beam quality, than end-pumped lasers. Although side-pumping of round cross-section laser rods is possible, many side-pumping schemes utilize a rectangular slab of laser active solid-state material, where the laser active slab has an either square or rectangular cross section. Laser active slab geometries may provide for heat removal from the laser medium such that the thermal gradient established by the heat removal occurs primarily in a single direction. This configuration allows a linearly-polarized laser beam to be amplified in the laser active slab, with the polarization of the laser beam either parallel or perpendicular to the thermal gradient, and without objectionable effects due to thermal stress-induced birefringence.
Several diode-side-pumped laser gain module, or amplifier, schemes have emerged in recent years that enable matching of a TEM00 laser beam or resonator mode to the optically pumped volume of the solid-state laser medium, as is typically required for efficient operation. For example, Eggleston et al. [“The slab geometry laser—Part I: Theory,” J Quantum Electronics, vol. 20, pp. 289-301 (1984)] describe a side-pumped zig-zag slab scheme in which the beam being amplified is reflected multiple times, via total internal reflection (TIR), at each of two parallel faces of the slab. Diode pump power is injected into the slab through the same two faces configured for reflecting the amplified beam via TIR. These two surfaces are also used for cooling the slab. A zig-zag path taken by the amplified beam helps to average out, or mitigate, spatial distortion effects on the amplified beam profile that pump-induced and cooling-induced non-uniformities in the slab might otherwise have.
As taught by the Eggleston reference, the same two parallel faces of the laser active slab are used for pumping through, heat removal, and reflecting the amplified beam in the laser active slab. These requirements significantly complicate the design and fabrication of such laser active slab designs. While the use of TIR to establish a zig-zag beam path helps to prevent amplified spontaneous emission (ASE) and parasitic oscillation problems, it also limits the number of reflections, and therefore the single-pass gain length, the designer can achieve along a given length of laser active slab medium.
There is also disclosed in U.S. Pat. No. 5,271,031, issued to Baer, a zig-zag slab design in which diode pump light is injected through the same parallel slab side faces used for guiding an amplified beam along a zig-zag path in a laser active slab. Baer '031 further teaches a high-efficiency, mode-matched, solid-state laser with transverse pumping and cascaded amplifier stages. Thin-film dichroic optical coatings, highly reflecting at the laser wavelength and highly-transmitting at the diode pump wavelength, are disposed on the parallel lateral faces. TIR was not used in establishing a zig-zag beam path through the laser active slab. The slab was cooled through transverse faces (i.e., top and/or bottom surfaces of the laser active slab), and not through the lateral faces used for diode pumping and zig-zag beam reflection.
In the Baer '031 design, a single 1-D array diode bar pump source for pumping through one lateral face, or two diode bars for pumping through both parallel lateral faces, are positioned very close (about 0.45 mm) to the laser active slab so as to minimize divergence of the emitted beams from the individual emitters in the diode bar (i.e., beamlets) before entering the laser active slab medium. This configuration provides for distinct individual beamlets that enter and pump the laser active slab material, and thus produce a corresponding 1-D array of discrete gain regions in the lateral side of the laser active slab. The array of diode-pumped gain regions is in a one-to-one correspondence with the pattern of diode emitters in the diode bar pump source, and has the same spacing (or pitch) between gain regions as the pitch of the diode emitters on the laser bar. That is, for a uniformly spaced array of diode emitters, the array of discrete gain regions in the slab is likewise uniformly spaced with the same pitch.
The apexes of a zig-zag path taken by an amplified beam through the laser active slab are aligned to spatially overlap with all of the diode-pumped gain regions corresponding to the emitter positions of the diode laser bar. With this “tightly folded” configuration, Baer was able to demonstrate a laser amplifier gain as high as 15 dB (i.e., an amplification factor of 32) for a single pass through the laser amplifier, when pumping the laser active slab with a 10 W diode bar. However, using the design taught by Baer '031 presents a challenge in both initially achieving and maintaining the tightly-folded beam alignment with the diode emitters. This complicates practical applications of the Baer '031 design, especially when applied to a moderate- or high-volume production setting. Moreover, unwanted amplified spontaneous emission (ASE), or parasitic oscillation, generated at the lasing wavelength of the laser active medium can build up in the laser active slab between the parallel high-reflection lateral surfaces. This unwanted ASE generation can dramatically reduce amplifier gain unless certain special precautions are taken for prevention of the parasitic oscillations and ASE.
Several methods are described in Baer '031 for preventing parasitic lateral oscillations and ASE. One method involves producing micro-patterned coatings on the parallel faces of the laser active slab, with the coatings having alternating high-reflection (HR) and anti-reflection (AR) coating regions. During operation of the laser amplifier, a zig-zag beam path reflects off the parallel sides of the laser active slab at the HR regions of the coatings. The HR regions have the same pitch along the length of the laser active slab as the diode emitter pitch along the length of the diode bar. An HR region on one of the parallel faces of the laser active slab is directly opposite an AR region on the other parallel face, thereby preventing, in theory, the buildup of lateral parasitic oscillations or ASE across the width of the laser active slab. Another method for suppressing lateral parasitics described in Baer '031 involves coating both parallel surfaces with an HR coating, and then selectively etching away the coating so that an HR region on one of the parallel faces is directly opposite a region on the other parallel face in which the HR coating has been etched away. Such micro-patterned coatings are difficult and expensive to fabricate, and may not be considered practical for production in moderate or high volume applications.
A third method for suppressing lateral parasitics, described in Baer '031, is to “slightly wedge” the two nominally parallel faces. The reference does not quantify the magnitude of the wedge angle except to state that, if the wedge angle is too large, then a diode bar with non-uniform spacing between the diode emitters is needed to maintain mode matching between diode emitter positions and apex positions of the zig-zag beam path. This is because the reflecting surfaces would no longer be parallel, and the apex points of the zig-zag path would therefore be spaced non-uniformly along the length of the laser active slab. In a scientific article related to Baer '031, [Baer T M, et al., “Performance of diode-pumped Nd:YAG and Nd:YLF lasers in a tightly folded resonator configuration,” J Quantum Electronics, vol. 28, pp. 1131-1139 (1992)], a wedge angle of 0.6 milliradians (i.e., 0.035 degrees or 2 arc-minutes) was mentioned as “probably” adequate to suppress lateral parasitics, but small enough to maintain mode matching at uniformly spaced diode emitter positions. However, it has been demonstrated that the 0.6 milliradian lateral wedge angle cited in Baer '031 is not adequate to suppress lateral parasitics and ASE when pumping Nd-doped 1064-nm laser slabs with high power levels of 40 W or more.
The shortcomings of the design taught by Baer '031 are also noted in U.S. Pat. No. 5,651,021 issued to Richard et al. Richard et al. '021 observe that, in the tightly folded design taught by Baer '031, “the maximum power obtainable from the design is limited by super-radiance (ASE) occurring between the parallel opposite reflective coatings. Furthermore, the multiple-pass optical path is quite complicated due to the requirement to match the reflection points to the active areas of the laser diode”.
Richard et al. '021 disclose a diode-side-pumped zig-zag slab laser design that employs a five-sided slab. In this design, total internal reflection (TIR) is used to guide the beam or mode being amplified along a zig-zag path that makes two passes along the length of the laser active slab. The laser active slab is diode-pumped through the two longest (and parallel) sides of the laser active slab, which also reflect the zig-zag beam path. A third side of the laser active slab, oriented perpendicularly to the two parallel long sides, provides another TIR reflection that sends the beam back through the laser active slab for a second zig-zag pass, reflecting again from the parallel lateral sides. Two Brewster-angle faces at one end of the laser active slab provide entrance and exit facets for the laser beam or mode being amplified. An overall two-pass, or round trip, gain length of about 76 mm is achieved with a laser active slab that is about 15 mm long and 3 mm wide, according to a related paper by Richard and McInnes [“Versatile, efficient, diode-pumped miniature slab laser,” Optics Letters, vol. 20, pp. 371-373 (1995)].
The slab in the design disclosed in Richard et al. '021 is relatively easy and inexpensive to fabricate because it has no coated or curved surfaces. Lateral parasitic oscillation and ASE are prevented by virtue of using TIR to create the zig-zag beam path through the laser active slab. Although a 76 mm of round trip gain length for a 3 mm wide by 15 mm long laser active slab is advantageous, such gain length may be substantially shorter than what can be achieved if, for example, high-reflection coatings and steeper incidence angles at the reflecting faces are used to confine the two-pass zig-zag beam path in the laser active slab.
U.S. Pat. No. 5,774,489 issued to Moulton et al. discloses a diode-side-pumped laser active slab amplifier in which the beam being amplified enters and exits through the longitudinal end faces of the laser active slab. The beam being amplified does not reflect off the lateral side surfaces through which diode pump light is injected. Rather, the amplified beam is directed with mirrors, positioned at the longitudinal ends of the laser active slab, to make multiple zig-zag passes through the laser active slab along its greatest dimension. The end mirrors used to achieve the zig-zag beam configuration may be external mirrors, or, alternatively, end mirrors may be coated directly onto a portion of each end face of the laser active slab, leaving window sections on end faces for the amplified beam to enter and exit.
As taught by Moulton et al. '489, longitudinal parasitic oscillations or ASE can build up between the nominally parallel end mirror surfaces used to achieve the zig-zag beam configuration. Furthermore, this and other gain module designs in which the amplified beam enters and exits the laser active slab at the end faces are difficult to multiple-pass without using a Faraday rotator. Also, because of the multiple segmented coatings that may be required, the laser active slab design taught by Moulton et al. '489 can be difficult and expensive to fabricate, especially if the end mirrors used for creating the zig-zag beam path are coated directly onto the slab end faces.
It can be appreciated by one skilled in the relevant art that most prior art devices that use a zig-zag slab laser design employ total internal reflection (TIR) at parallel side faces to establish a zig-zag beam path through the laser active slab. Because the internal angle of incidence of the zig-zag beam path at the reflecting surfaces must be larger than a specified minimum angle, a TIR design limits the overall length of the zig-zag beam path and, therefore, the laser gain-per-pass that can be achieved in the gain module or amplifier. Designs that employ thin-film coatings to establish a zig-zag path through the laser slab enable steeper angles of incidence, more reflections, longer zig-zag path lengths and, therefore, potentially higher amplifier gain-per-pass, but only if build up of unwanted parasitic oscillations and amplified spontaneous emission (ASE) are prevented. It does not appear that prior art devices employing thin-film coatings to establish a zig-zag beam path have successfully dealt with the problem of unwanted parasitics and ASE. In addition, for some prior art laser amplifier devices using a two-pass amplifier configuration to increase overall gain, especially for devices that employ thin-film coatings, a Faraday isolator or rotator device must be included to separate the input and two-pass output beams. Furthermore, achieving three or four zig-zag passes in such thin-film-coated slabs has often not been possible or practical.